Processors such as CPUs (central processor units), DSPs (digital signal processors) and microprocessors, and other electronic devices require accurate voltage supplies capable of supplying large amounts of current and maintaining tight voltage regulation when subject to very fast transients. Multiphase, high frequency buck converters are capable of supplying high current, high accuracy power to such electronic devices. Power supply efficiency, i.e. the ratio of output power delivered to the load and input power supplied by the input voltage, is a critical performance parameter for both energy efficiency and thermal management of power supply components. Typically, systems focus on both efficiency at maximum load, and maximum efficiency, which typically occurs at about 20-50% of maximum load.
Under light load conditions, the load enters a low activity state and the current drops to less than about 5% of maximum load. The efficiency under such light loads is significantly below the maximum efficiency and continues to decrease as the load current is further reduced. Efficiency at light load conditions is increasingly becoming a greater concern because conventional systems are not optimized for efficiency at light load conditions and usage models indicate a significant amount of computing resources are idle most of the time. This is problematic for both servers where thermal management of data centers is a key consideration in cost of operation, and for desktop and notebook computers and portable electronic devices where battery life and quiet operation are highly valued.
Conventional controllers for power converters employ control schemes that allow the switching regulator to seamlessly switch from CCM (continuous current mode) to DCM/PFM (discontinuous current mode/pulse frequency modulation) operation. However, light load efficiency of the switching regulator is limited due to the large power components (e.g. drivers and power transistors) and low value inductances needed to support high current multiphase operation. For example, multiphase high frequency buck converters are suitable for supplying power to electronic devices such as processors at maximum load conditions, but efficiency at light load is poor if multiphase CCM operation is maintained. Multiphase converters can operate with improved efficiency at light load conditions if phase dropping is used to deactivate all but a single phase of the converter, and that phase is operated in DCM with PFM (pulse frequency modulation) control. However, even with single phase PFM efficiency at very low load currents suffers because the power stage components (e.g. driver and transistors) are typically quite large with high parasitic capacitance and the inductance is low to support the high current and transient requirements of the electronic device.